Digitally measuring scopes using a high resolution encoder

ABSTRACT

A system for determining a dimension of a detail including an optical scope for producing an image of the detail, an image scaling device for providing a scaled image size, and a processor. The system also includes a video camera which is either within the optical scope or external to the optical scope. The optical scope includes a focusing device for adjusting a focal position of the image of the detail and a device for providing a focus position signal based a position of the focusing device. If the system includes an external video camera, the optical scope also includes a viewer for passing the image of the detail to a plane outside of the optical scope. The video camera optically is coupled with the viewer of the optical scope or with the image of the detail and produces a video signal of the detail from the image of the detail. The processor converts the focus position signal into an object distance signal, or a magnification signal, or both, and determines the dimension of the detail based on the scaled image size and based on the object distance signal, or the magnification signal, or both.

CONTINUATION APPLICATION INFORMATION

This application is a continuation of U.S. patent application Ser. No.08/502,984, filed Jul. 17, 1995, now U.S. Pat. No. 5,801,762, which is acontinuation-in-part of U.S. patent application Ser. No. 08/365,636,filed Dec. 28, 1994, now U.S. Pat. No. 5,573,492.

BACKGROUND INFORMATION

The present invention concerns an arrangement, which includes a scope,such as a rigid borescope, a flexible fiberscope, or a videoscope forexample, for accurately measuring observed objects, object details, andobject defects. The arrangement of the present invention is moreaccurate than known systems, yet is simple to use.

Scopes, such as flexible videoscopes and fiberscopes have been used toobserve the interior of the body during diagnostic procedures orsurgery. Scopes, such as rigid borescopes, have been used to observe andinspect manufactured parts otherwise inaccessible to the eye. Althoughsuch scopes have an almost limitless number of applications, thefollowing example illustrates their value.

A gas turbine engine includes a series of compressor and turbine blades,any one of which may become damaged. Although the first and last stagecompressor/turbine blades of a gas turbine engine can be inspecteddirectly, other intermediate stage compressor/turbine blades cannot bedirectly inspected. In the past, to inspect these intermediate stagecompressor/turbine blades, the engine had to be disassembled until theintermediate stage compressor/turbine blade could be directly inspected.However, more recent gas turbine engines are provided with apertures (orborescope ports) provided at critical areas. These borescope portspermit the intermediate stage blades to be inspected using a borescope.

The borescope includes a long, thin, insertion tube having a lens systemat its distal end and a viewing means at its proximal end. When theinsertion tube of the borescope is inserted into a borescope port of thegas turbine engine, the lens system at its distal end relays an image ofan otherwise inaccessible intermediate compressor/turbine blade to theviewing means at the proximal end. The focus of the image (in somemodels) can be adjusted by control knobs at the proximal end of theborescope. Hence, as illustrated by this example, a borescope permits anintermediate compressor/turbine blade of a gas turbine engine to beinspected without needing to disassemble the engine.

Besides being used to indirectly inspect parts which cannot be inspecteddirectly, borescopes can also be used to measure the size of defects onthe part. For example, U.S. Pat. No. 4,980,763 (hereinafter “the '763patent”) discusses a system for measuring objects viewed through aborescope. The system discussed in the '763 patent projects an auxiliaryimage, such as a shadow, onto the object being viewed. Changes in theposition or size of the auxiliary image correspond to the distancebetween the object being viewed and the borescope. The image isdisplayed on a monitor having a magnification and object distance scaleoverlay on the screen. The size of the object on the screen is measuredwith vernier calipers, or electronically with cursors. This size is thendivided by the magnification which is determined by observing where theauxiliary image falls on the magnification overlay. Unfortunately, thesystem discussed in the '763 patent requires a user to manuallydetermine the magnification factor based on the position of theauxiliary image on the display screen.

U.S. Pat. No. 4,207,594 (hereinafter “the '594 patent”) discusses asystem in which the dimensions of a defect are determined based upon amanually entered field-of-view value and a ratio of second crosshairs,arranged at edges of a defect image, to first crosshairs, arranged atedges of the field of view. Unfortunately, the system discussed in the'594 patent requires probe penetration values to be manually read from ascale on the probe barrel for determining the field-of-view. Since suchscales do not have fine gradations and since they must be manually read,errors are introduced.

U.S. Pat. No. 4,820,043 (hereinafter “the '043 patent”) discusses atechnoscope for determining the length of a defect. The technoscopeincludes a graduated scale which is displaceable in a directiontransverse to the endoscope axis. The graduated scale is mechanicallycoupled with a detector which produces an electrical signal based on thetransverse displacement of the graduated scale. The distance to thedefect is determined by (i) observing the object image at a firstterminal position of a fixed stroke Z of the endoscope, (ii) noting theintercept of the object image on the graduated scale, (iii) axiallydisplacing the endoscope by the fixed stroke Z, and (iv) transverselydisplacing the graduated scale until the defect image intercepts it atthe same point as before the axial displacement. A calculator uses theelectrical signal from the detector and a known focal length ofendoscope to determine the object distance. The size of the defect canbe similarly determined. The technoscope of the '043 patent alsoincludes a swing prism with a detector for determining its angularposition.

Unfortunately, the scope of the '043 patent requires that the focallength of the endoscope be known ahead of time and requires twomeasurements. Moreover, since the distance between the two measurementsmust be fixed, the scope must be fixed with respect to the object duringthe two measurements. Furthermore, limitations in the gradations of thegraduated scale limits the accuracy of the readings. Also, by manuallyreading the intercept point of the defect on the graduated scale, errorsare introduced.

U.S. Pat. No. 4,702,229 (hereinafter “the '229 patent”) discusses atechnoscope for measuring an object. The technoscope includes an innershaft which is axially displaceable with respect to an outer shaft. Ameasuring scale is provided in the inner shaft. The measurement of theobject is determined by (i) placing an edge of the object image on themeasuring scale, (ii) fixing the technoscope with respect to the object,(iii) axially displacing the inner shaft by a fixed distance, and (iv)observing how many scaler divisions the object image moved on themeasurement scale. The object size is determined based on a known systemfocal length, the length of the displacement, and the number of scalesmoved by the object. The '229 patent is similar to the '043 patentexcept that with the '043 patent, the graduate scale is transverselyrepositioned such that the object intercepts it at the same point andthe transverse position is determined with a mechanical detector.Therefore, the device of the '229 patent suffers the same drawbacks asthe '043 patent, namely, (i) the focal length of the technoscope must beknown, (ii) two measurements are needed, during which the technoscopemust be fixed with respect to the object, (iii) limitations in thegradations of the measurement scale introduces errors, and (iv) themeasurement scale must be manually read.

Known devices also use a magnification scale ring arranged adjacent to afocusing control ring having an indicator RV, for determining themagnification of the scope. Based on the position of the indicator ofthe focusing control ring with respect to the magnification scale, themagnification of the scope at that object distance is determined.Unfortunately, similar to the probe penetration knob in the systemdiscussed in the '594 patent, such devices require magnification valuesto be manually read from a scale on the magnification barrel. Since suchscales do not have fine gradations and since the magnification valuesmust be manually read, errors are introduced. Even if the scale had finemarkings, its diameter would have to be huge to have thousands ofdistinct “markings.”

When such a scope is equipped with a graticule and a diopter focuscontrol, this known device can also be used to determine the size of aviewed object. A graticule is a scale etched into a surface of atransparent glass plate included in the optical system. The diopterfocus control is used to focus the graticule scale. An object is thenfocused by means of the focus control. Based on the number of graticulescovered by the object image and based on the magnification level, theobject size is determined. Unfortunately, the graticule scale ismanually read which introduces errors. Manually reading the number ofgraticules covered by the object image also fatigues the user's eye.Moreover, since the number of markings on the graticule is limited, theaccuracy is also limited. Furthermore, this method is inaccurate becausethe eye will accommodate an “out of focus” focus barrel position.

U.S. Pat. No. 4,558,691 (hereinafter “the '691 patent”) discloses anendoscope in which an actual size of an observed object, a magnificationof the scope, and an object distance can be determined based on apositional relationship between an indicating index and a stationaryreference index. The indicating index is formed on a glass plate whichmoves up and down, perpendicular to the optical axis, as the lens barrelof the optical system moves back and forth along the optical axis.Unfortunately, as with the devices discussed above, the indicating indexincluded in the device described in the '691 patent does not have finegradations and must be manually read. This not only permits errors to beintroduced, but also fatigues the eye of a user.

Japanese Patent Publication No. 5-288988 (hereinafter “the '988publication”) discusses the use of an encoder for determining changes inthe magnification of a zoom lens system. However, this system is to beused for viewing objects at a fixed distance. That is, the encoder inthe '988 publication determines changes in magnification of the scopebut cannot determine the initial magnification of the scope and cannotdetermine object distance.

In view of the above described problems with existing scope measurementsystems, a system for automatically measuring objects, with highresolution, is needed.

SUMMARY OF THE INVENTION

The present invention fulfills the above mentioned need by providing anarrangement including a scope having a focusing mechanism to which ahigh resolution encoder is coupled. The encoder sends a signal,corresponding to the position of the focusing mechanism, to a processor.The image from the scope is also sent to the processor. A processorexecuted program correlates the encoder signal to object size and/ormagnification.

The encoder may be either a relative encoder or an absolute encoder, andmay encode optically, electrically, and/or magnetically. However, in apreferred embodiment of the system of the present invention, the encoderis a relative, optical, encoder.

In a first embodiment of the present invention, the scope is a swingprism rigid borescope which allows the user to change the scope'sdirection of view from the scope body. The encoder is preferably arotational optical encoder. The image from the scope is preferably sentto the video processor via a camera mounted to an eyepiece. In anembodiment of the system of the present invention using a relationalencoder, the relational encoder increments and decrements an initialcount based on adjustments of the focusing device. The initial count isset when the focusing device is placed in a predetermined home position.

In a second embodiment of the present invention, the scope is a flexiblefocusing fiberscope having a focusing lens system at a distal end and afocusing control at a proximal end. The focusing control at the proximalend actuates at least one lens of the focusing lens system at the distalend by means of a control cable or by means of a fine pitch flexiblescrew. The encoder may be a linear encoder located at the distal end formeasuring the linear movement of the at least one lens of the focusinglens system, or it may be an encoder located at the proximal end formeasuring a movement of the focusing control. In a third embodiment, thescope is a flexible video fiberscope having a focusing lens system atthe distal end and a focusing control at the proximal end.

Optical encoders may include rotational encoders or linear encoders. Therotation encoders include an encoder disk, a light source and adetection device. The encoder disk has apertures, or reflective andnon-reflective regions, arranged around its circumference, and ismechanically coupled with the focusing device such that it rotates whenthe focusing device is adjusted. The light source directs light towardsa first side of the encoder disk. When an encoder disk having aperturesis used, the detection device is arranged on a second side of theencoder disk, and generates a pulse when light from the light sourcepasses through an aperture of the encoder disk. On the other hand, if anencoder disk with reflecting and non-reflecting regions is used, thedetection device is arranged on the first side of the encoder disk andgenerates a pulse when light from the light source is reflected by theencoder disk. A linear encoder is similar to the disk encoder except ithas a strip having a plurality of spaced apertures or a plurality ofreflecting and non-reflecting regions, and is mechanically coupled withthe focusing device such that it is linearly translated when thefocusing device is adjusted.

The present invention provides a system for determining a dimension of adetail. The system at least includes an optical scope and a processor,and may also include a video camera, a display device, and a detailmarking device. The optical scope produces an image of the detail andincludes a focusing device and an encoder. Borescopes and fiberscopesalso include a viewer. The focusing device adjusts a focal position ofthe image of the detail. With borescopes and fiberscopes, the viewerpasses the image of the detail to a plane outside of the optical scope.With videoscopes, a video signal is provided. The encoder provides afocus position signal based a position of the focusing device.

The video camera produces a video signal of the detail from the image ofthe detail. In borescopes and fiberscopes, the video camera is opticallycoupled with the viewer of the scope, while in videoscopes, the videocamera is internally mounted. The display device displays the detailbased on the video signal of the detail. The detail marking devicepermits at least two markers, each having a coordinate value, to bearranged on the display of the detail on the display device. Theprocessor converts the focus position signal from the encoder into anobject distance signal and then into a magnification signal. Theprocessor also determines the dimension of the detail based on thecoordinate values of the at least two markers and based on themagnification signal.

In a preferred embodiment of the system of the present invention, theoptical scope is a swing prism rigid borescope. The optical scope mayalso be a fiberscope or videoscope.

In a preferred embodiment of the system of the present invention, thevideo camera includes a charge coupled device which converts the imageof the detail into the video signal of the detail. Further, in thepreferred embodiment of the system of the present invention, theprocessor includes a first converter, such as a formula or a look-uptable, for converting the focus position signal into an object distancesignal, and a second converter, such as a formula or look-up table, forconverting the object distance signal from the first converter into amagnification signal. In the preferred embodiment, the processor alsoincludes a size processor for producing the dimension of the detailbased on the magnification signal from the second converter and based onthe coordinate values of the at least two markers.

In a preferred embodiment of the present invention, the optical scopeincludes a distal lens system, a focusing lens, a focus controller, andan encoder. If the optical scope is a borescope or fiberscope, it alsoincludes a viewer. The distal lens system produces an image of a detailwithin its field-of-view. If the optical scope is a borescope orfiberscope, the viewer passes an image of the detail produced by thelens system to a plane outside of the scope. The focusing lens islocated between the distal lens system and the viewer and can belinearly translated along its optical axis thereby permitting a focalposition of the image to be adjusted. The focus controller linearlytranslates the focusing lens along its optical axis, whereby differentpositions of the focus controller correspond to different positions ofthe focusing lens. If the optical scope is a videoscope, a video camera,such as a CCD for example, converts an image to a video signal at thedistal end. The focusing control actuates at least one lens in a lenssystem at the distal end. The encoder produces signals corresponding tothe different positions of the focusing lens being translated or of thefocus controller.

In an alternative embodiment of the present invention, the scopeautomatically focuses the image onto the video camera. In thisembodiment, a stepper motor can actuate at least one lens of thefocusing lens system. Alternatively, a user may make processor guidedmanual focusing adjustments. The user positions a cursor upon the videoimage of the detail to be measured. Alternatively, known methods ofpattern detection could be used. A number of samples of windows (i.e., apredetermined number of pixels around the area of interest) are takenwith the at least one lens of the focusing lens system at differentpositions. The at least one lens is translated by the stepper motor. Thesampled window with the maximum contrast, as determined by a knownmethod, is considered to be the most in focus. If there is more than onemaximum contrast, the processor can choose either (i) the near focusside position having a maximum contrast, (ii) the far focus sideposition having a maximum contrast, or (iii) the average focus positionof the maximum contrasts. This choice is predetermined. While any of thethree choices can be used, it must be used consistently and can be thebasis for system calibration.

In a preferred embodiment of the present invention, an opticaldistortion, based on an eccentricity of the image with respect to thefocusing lens system, is corrected by the video processor. Theeccentricity is determined based on the location of the image on thevideo camera and/or on the video monitor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view illustration of a rigid borescope.

FIG. 2 is an illustration of a marked focus barrel used in knownmeasuring scopes.

FIG. 3 is a cross-sectional side view of an ocular position rotationalencoder to be used with the system of the present invention.

FIG. 4 is a plan view of the ocular position rotational encoder shown inFIG. 3.

FIG. 5 is a perspective view illustrating a focus barrel having a spiralgroove.

FIG. 6 is a perspective view illustrating a borescope chassis having alongitudinal slot.

FIGS. 7a and 7 b are cross-sectional side views of the ocular rotationalencoder of FIG. 3 illustrating the difference in the positions of theelements of the encoder when viewing relatively distant objects andrelatively close objects.

FIG. 8 is a schematic diagram of an emitter end plate, a code wheel, andan encoder body of a rotational encoder that can be used in the systemof the present invention.

FIGS. 9a through 9 d are timing diagrams of outputs of the rotationalencoder illustrated in FIG. 8.

FIG. 10 is a functional block diagram of the system of the presentinvention.

FIG. 11 is a graph illustrating the relationship between magnificationand object distance for an exemplary borescope.

FIG. 12 is a look-up table for determining object distance from thecount produced by an exemplary encoder.

FIG. 13 is a flow diagram of a process for using the system of thepresent invention.

FIG. 14a is a schematic illustrating an objective fiberscope having adistally mounted encoder and having a proximal focusing cable tomechanically link a focus control with a focusing lens system.

FIG. 14b is a schematic illustrating an objective fiberscope having adistally mounted encoder and having a flexible fine pitch screw and afine pitch nut for mechanically linking a proximal focus control with afocusing lens system.

FIG. 14c is an alternative embodiment of FIGS. 14a and 14 b in which aproximally mounted encoder is used instead of a distally mountedencoder.

FIG. 15a is a schematic illustrating an objective videoscope having adistally mounted encoder and having a focusing cable to mechanicallylink a focus control with a proximal focusing lens system.

FIG. 15b is a schematic illustrating an objective videoscope having adistally mounted encoder and having a flexible fine pitch screw and afine pitch nut for mechanically linking a proximal focus control with afocusing lens system.

FIG. 15c is an alternative embodiment of FIGS. 15a and 15 b in which aproximally mounted encoder is used instead of a distally mountedencoder.

FIGS. 16a and 16 b are video display screens for illustrating a processfor determining magnification and radial distortion errors.

FIGS. 17 is a diagram which illustrates how magnifications aredetermined.

FIG. 18 is a flowchart of an automatic focusing procedure of the presentinvention.

FIG. 19 is a schematic diagram illustrating additional elements used inan automatically focusing scope.

DETAILED DESCRIPTION

The following is a description of an exemplary embodiment of the presentinvention which employs a rotational encoder with a borescope. Thisexample is not intended to limit the scope of the invention to rigidborescopes or to rotational encoders. Instead, the scope of theinvention is defined by the claims which follow the detaileddescription.

FIG. 1 is a side view illustration of a rigid borescope 1. The borescope1 includes a body 2 and a long, thin arm (or insertion tube) 3. Theinsertion tube 3 is connected, at a proximal end, to the body 2, has aninsertion length WL, and includes a lens system 10 at its distal end.The lens system 10 has a field-of-view defined by the angle FOV. As isillustrated in the partial side views of the insertion tube 3, differentlens systems 10 a through 10 d have different directions of view. Thedistal end of the insertion tube 3 includes a cap 9 for protecting theinsertion tube 3 from mechanical shocks resulting from inadvertentcollisions. In a preferred embodiment of the present invention, therigid borescope includes a swing prism, i.e., a prism which can bepivoted. The direction of view of such a swing prism can be adjustedfrom 45° to 120°. A sensor may be used to determine the angular positionof the prism. Any angular prism position deviating from 90° introducesoptical errors. These optical errors can be compensated for based on thesensor output.

The body 2 of the borescope 1 includes an orbital scan control 8, afocus control 4, a viewing means 11, and a light guide connector 6. Theorbital scan control 8 is used for rotating the insertion tube 3 forcapturing different views. The angular orientation of the insertion tube3 is indicated with an orbital scan direction indicator 7. The focuscontrol 4 permits the image gathered by the lens system 10 to be focusedat the viewing means 11. The light guide connection 6 permits a lightsource (not shown) to be coupled with the borescope. The light from thelight source may be carried by a fiber optic bundle, for example, to thedistal end of the insertion tube 3 to illuminate objects within thefield-of-view of the lens system 10. The viewing means 11 provides theimage captured by the lens system 10. An eyecup 5 may be connected tothe viewing means 11 for direct viewing. Alternatively, as illustratedin FIG. 10, a video camera 103 may be mounted to the viewing means bymeans of a viewing means-to-camera adaptor 116. The video camera 103 maytransmit a video signal to a video processor 108.

A “focusing borescope ” is a borescope with adjustable focus whichproduces a sharp image at a range of object distances. The narrowfield-of-view (e.g. FOV=20 degrees) results in a shallow depth-of-field(DOF). As a consequence, with focusing borescopes, positions of thefocus barrel correspond to object distances. Therefore, the focus barrelcan be calibrated and used as a range finder. Since the log of the scopemagnification and the log of the object distance have a linearrelationship, the actual size of an observed object can also bedetermined.

FIG. 2 is a marked focus barrel used in known measuring scopes, such asa focusing borescope for example. Such a marked focus barrel can beprovided on the body 2 of the borescope 1 of FIG. 1 for example. Thefocus barrel of FIG. 2 includes a magnification scale 22, a diopterfocus control 23, a graticule orientation control 24 and a focusingcontrol 21 having an indicator 211. When the observed image is focused,the position of the indicator 211 of the focusing control 21 withrespect to the magnification scale is used to determine themagnification of the image. As mentioned above, the log of the objectdistance is linearly related to the log of the scope magnification.Therefore, once the magnification of the image is determined, the objectdistance can be derived. Furthermore, as discussed in the Background ofthe Invention above, when the graticule (i.e., a scale etched into asurface of a coverglass included in the optic system) is focused withthe diopter focus control 23, the size of the object being viewed can bedetermined based on the number of graticules occupied by the image ofthe object and based on the magnification of the scope.

Unfortunately, a number of error sources are introduced when using ascope with a marked focus barrel to determine the object distance, i.e.,as a range finder. First, the resolution of the magnification valuereading is limited by the gradations of the magnification scale. Also,there is a very practical limit to the number of graticule markingswhich limits the accuracy. Second, the magnification scale 22 and thenumber of graticules covered by the focused image must be manually readby a user. Third, manual calculations and confusing multiple stepprocedures introduce the possibility of human errors. Furthermore, thepoor “ease of use ” of scope measuring systems employing marked focusbarrels and the experience of eye fatigue when using such scopes haslimited their acceptability. More importantly however, is that the eyewill accommodate slightly “out of focus” images. Furthermore,differences in the eyesights of different users will lead to differentmagnifications needed to focus the image. That is, users havingdifferent eyesights will result it different focus positions for theobject being viewed.

FIG. 3 is a cross-sectional side view, and FIG. 4 is a partial planview, of an ocular position rotational encoder which can be used in thescope measurement system of the present invention. The ocular rotationalencoder includes an encoder pickup 30 (described in more detail withreference to FIG. 8), an encoder mounting plate 31, a borescope chassis32, an encoder disk 33, an encoder disk hub 34, a carrier with an ocularlens 35, a ocular position pin 36, a viewing means 37, a focusing knob38, and a focusing barrel 39.

The ocular position rotational encoder can be mounted to a rigidborescope by means of the mounting plate 31. The viewing means 37 is aneyepiece. However, as illustrated in FIG. 10, in the preferredembodiment of the present invention, a video camera 103 is mounted tothe viewing means 37 with a camera adaptor 116. The video camera 103transmits a video signal to a video processor 108 which displays thecaptured image 110 on a display monitor 109. This arrangement permitsthe image captured to be more accurately focused than would be possiblewith a human eye at the viewing means 37 because the video camera 103includes an imaging means, such as a charge coupled device (CCD) 104,which is maintained at a fixed distance from the viewing means 37.Accordingly, the video camera 103 provides a “hard plane of focus” whichis advantageous when compared with the different focus positionsresulting from different users having different eyesights as discussedabove and resulting from the eye's ability to accommodate slightly“out-of-focus” images.

When the focusing knob 38 is rotated about the optical axis, the ocularlens carrier 35 moves up or down as indicated by the arrows. By viewingthe image on the display monitor, the focusing knob 38 can be used toprecisely focus the image transmitted through the ocular lens held inthe carrier 35. This is done as follows.

The focusing knob 38 is mechanically coupled with the focusing barrel39. In a preferred embodiment of the present invention, the focusingknob 38 is directly attached to the focusing barrel 39. As shown in FIG.5, the focusing barrel 39 is cylindrical and has a spiral groove 51 cutinto its inner surface. The ocular position pin 36 fits into the spiralgroove 51. The inner surface of the focusing barrel 39 has a slightlylarger diameter that the outer surface of the borescope chassis 32thereby permitting the focusing barrel 39 to rotate about the borescopechassis 32. As shown in FIG. 6, the borescope chassis 32 has alongitudinal slot 61 through which the ocular position pin 36 projects.Accordingly, when the focusing knob 38 is rotated, the directlyconnected focusing barrel 39 also rotates about the borescope chassis32. The spiral groove 51 on the inside surface of the focusing barrel 39causes the ocular positioning pin 36 to ride up or down in thelongitudinal slot 61 of the borescope chassis 32. Since the ocularpositioning pin 36 is attached to the ocular lens carrier 35, the ocularlens can be moved up and down by rotating the focusing knob 38.

FIGS. 7a and 7 b illustrate the relative positions of the ocular lenscarrier 35 and of the ocular positioning pin 36 within the groove 51 onthe inside surface of the focusing barrel 39, for an object relativelyfar from the borescope and for an object relatively close to theborescope, respectively. As illustrated in FIG. 7a, when the objectdistance is relatively large, the ocular lens carrier 35 is positionedfar from the viewing means 37 and the ocular positioning pin 36 islocated high in the spiral groove 51 on the inside surface of thefocusing barrel 39. On the other hand, as illustrated in FIG. 7b, whenthe object distance is relatively small, the ocular lens carrier 35 ispositioned close to the viewing means 37 and the ocular positioning pin36 is located low in the spiral groove 51 on the inside surface of thefocusing barrel 39.

As illustrated in FIG. 3, the focusing barrel 39 is also mechanicallycoupled with and preferably directly connected to, the disk hub 34 ofthe encoder disk 33. In a preferred embodiment of the present invention,as shown in FIG. 4, the encoder disk 33 has a number of slots or holes40 (only three of which are shown for clarity) arranged around theencoder disk 33, spaced in equal angular increments. The encoder pick up30, described in detail with reference to FIG. 8, determines the angularrotation of the encoder disk 33 by counting pulses received at thepickup 30.

The optical encoder which produces pulses corresponding to changes inthe focal position, is a so-called “relational encoder”. That is, aninitial condition (i.e., an initial count) must be determined before thecount is incremented or decremented. This initial condition isdetermined by locating the focal position to a predetermined “home”position for which the count is known or reset by the electronics,typically to zero. The known or reset count is then incremented and/ordecremented when the focal position is moved from the “home” position.The “home” focal position is preferably located at at least one of thetwo extreme focal positions. Alternatively, a home position can bedetermined with an “indexing channel” as described below.

As an alternative to such “relational encoders,” which increment and/ordecrement a known count when the focal position is moved from apredetermined position, an “absolute encoder,” which includesinformation about its absolute angular (or linear) position may be used.

FIG. 8 is a schematic diagram of the encoder optical pick-up 30. Theencoder optical pick-up 30 includes an emitter end plate 81 arrangedadjacent to a first surface of the encoder disk 33 and an encoder body82 arranged adjacent to a second surface of the encoder disk 33.

The encoder end plate 81 includes a series of light emitting diodes 83 athrough 83 c for emitting light which is collimated by lenses 84 athrough 84 c, respectively. These collimated light beams are directedtowards the first surface of the encoder disk 33.

The encoder body 82 includes a phase plate 85, lens pairs 86 a through86 c, and detection elements 87 a through 87 c for three (or two in analternative embodiment) channels. Each of the channels 87 a through 87 cincludes an integrated circuit having two photodiodes each having itsown amplifier 88 a through 88 c. The amplifiers 88 a ₁ through 88 c ₁are electrically coupled with a non-inverting input of comparators 89 athrough 89 c, respectively, while the amplifiers 88 a ₂ through 88 c ₂are electrically coupled with an inverting input of the comparators 89 athrough 89 c, respectively.

The collimated light beams from lenses 84 a through 84 c must passthrough a slit 40 in the encoder disk 33 and through an opening in thephase plate 85 to reach lens pairs 86 a through 86 c, respectively. Theapertures in the phase plate 85 are positioned such that, for eachphoto-diode/amplifier pair 88 a through 88 c, a light period on onedetector always corresponds to a dark period on the other. Accordingly,the output state of the comparators 89 a through 89 c changes when thedifference of the two photo currents produced by thephoto-diode/amplifier pairs 88 a through 88 c, respectively, changessign.

The phase plate 85 is also arranged so that the channel 87 a is 90degrees out of phase (i.e., in quadrature) with the channel 87 b as isshown in the timing diagram of FIGS. 9a and 9 b. This phase differencepermits the direction of rotation to be determined by observing whichchannel is the leading waveform.

The channel 87 c is an optional channel for performing an indexingfunction. Specifically, the channel 87 c generates an index pulse foreach rotation of the encoder wheel 33. This indexing channel can be usedto define a “home” focal position having a known count, therebyproviding an initial condition, i.e., an initial count, for this“relational” encoder.

FIG. 10 is a functional block diagram of the system of the presentinvention. An image captured by the borescope 1 is provided to a videocamera 103 which is coupled with the viewing means 37 of the borescope 1via a viewing means-to-camera adaptor 116. The video camera 103 includesan imaging device 104, such as a charge-coupled device (CCD) forexample, for converting the optical signal to an analog video signal.The analog video signal is supplied to a video processor 108. The videoprocessor 108 provides a video signal of the object to a video displaymonitor 109 via a digital video frame capturing device 115. The focusingknob 38 is adjusted until an image of the object 110 being observedappears in focus on the video display monitor 109. For objects orientedat an angle to the plane of the optical system of the scope, multiplepoints or regions of the object can be separately focused.

While the focusing knob 38 is being adjusted, the encoder 30 generatespulses corresponding to the angular rotation of the focusing knob 38.The encoder 30 transmits the pulses of channels 87 a, 87 b, and 87 c toa counter 101. The counter 101 increments or decrements the count,depending upon whether the focusing knob 38 is being rotated clockwiseor counter-clockwise, thereby forming a digital count value. An initialcount is generated when the focus position is at “home” position asdiscussed above. The digital count value is stored in a buffer 102.

When the object being observed 110 appears in focus on the video displaymonitor 109, the user actuates a switch 112, such as a key of a keypad,to “freeze” the image. When the switch 112 is actuated, the digitalcount stored in the buffer 102 is read out and provided as an input to acount-to-object distance converter 106, and the video frame stored inthe digital video frame capture 115 is read out and provided to thevideo display monitor 109. If multiple points or regions of an angledobject are separately focused as provided above, the countscorresponding to the separate focus positions are averaged. In a furtherembodiment of the present invention, described more fully below,multiple points of a feature, such as a ding, can be separately focusedand the measured point distances used to determine the depth (or height)of the feature or of parts of the feature.

The count-to-object distance converter 106 can be implemented as apredetermined formula for converting the digital count to an objectdistance. For example:

log x=a(log y)² −b (log y)+c

wherein

x=object distance

y=encoder count

a,b,c=constants

Alternatively, the object distance can be determined from the count bymeans of a look up table including empirically determined data. FIG. 12shows an example of such a look up table. If a look up table is used forconverting the digital count to an object distance, an interpolationroutine is preferably also used for determining object distances whenthe digital count falls between count values listed in the look uptable.

The object distance determined by the count-to-object distance converter106 is provided as an input to an object distance-to-magnificationconverter 107. Similar to the count-to-object distance converter 106,the object distance-to-magnification converter 107 can be implemented asa predetermined formula for converting the object distance to amagnification value. Alternatively, the magnification value can bedetermined from the object distance by means of a look up table and anoptional interpolator. As the graph shown in FIG. 11 illustrates, alinear relationship exists between the log of the magnification and thelog of the object distance.

The system of the present invention also can automatically compensatefor changes in the magnification from system to system due todifferences in the video cameras (such as CCDs for example) and due todifferences in the coupling between the eyepiece and the video camera.This automatic compensation eliminates the need to calibrate themagnification for each scope or system. The system of the presentinvention can also compensate for optical distortions due to imageeccentricity. All optical systems distort. The main component of opticaldistortion for scopes results from images that do not pass through thecenter point of the lenses of the lens system. Specifically, an opticaldistortion is a function of the radial distance from the center of thelens to the point at which the image passes, i.e., the opticaldistortion is greater when the image passes through the edges of thelens than when the image passes through the center of the lens.

The system of the present invention automatically compensates forvariations in magnification and for optical distortion as follows. FIG.16a illustrates a screen of the display 109 showing the image of thedefect 110. Since the actual image does not fill on the entire area ofan imaging device (such as a CCD 104 for example) of the video camera103, the scope image 161 does not fill the entire screen of the display109. The size of the scope image 161 depends, at least in part, on theoptical coupling of the image to the imaging device. A user can move andadjust the diameter of the circle 162 with an input device, such as thecursor input control 113, for example. The user moves and adjusts thecircle 162 such that it coincides with the scope image 161 on thedisplay. (See FIG. 16b). A processor, such as the size processor 114,compensates the magnification of the optical scope 1—video camera 103combination based on the diameter of the circle 162. FIG. 17 is adiagram which illustrates the magnification compensation. Once theobject distance (OD) is determined, the actual size of the diameter ofthe optical scope (D_(OS)) can be determined since the field of view(FOV=20) is known. Specifically D_(OS)=2X=2 [OD(tanθ)]. Themagnification is then determined based on the ratio of the diameter ofthe circle 162 (when it coincides with the scope image 161) over D_(OS).

FIG. 16a also illustrates a crosshair 164 which can be moved by a userwith an input device, such as the cursor input control 113. As shown inFIG. 16b, the user can position the crosshair 164 at the center of thescope image 161 so that an eccentricity “E” from the center 165 of thedisplay can be determined. A processor, such as the size processor 114for example, can compensate for optical distortions based on theeccentricity “E”.

The magnification compensation and the optical distortion compensationmay be separately provided. However, if both are provided, the presentinvention preferably combines the crosshair 164 to coincide with thecenter of the circle 162 so that the image size and eccentricity can bedetermined based on a single user input. Alternatively, a processorexecuted program can determine the size of the scope image 161 and theeccentricity “E” automatically without requiring a user input. Theoptical scope may also include a non-volatile memory for communicatingstored calibration information with other components of the system.

Referring back to FIG. 10, after a user actuates the freeze image switch112, the user manipulates a cursor control input device 113, such as akeypad, a trackball, or a joystick, for example, to position at leasttwo crosshairs 111 a and 111 b at ends of the object 110 beingdisplayed. The coordinate positions of the crosshairs 111 a and 111 b,as well as the magnification factor, are provided as inputs to a sizeprocessor 114 which computes the length or size of the object 110 beingdisplayed. Alternatively, a graticule (or reticle) provided in theoptical scope can be used for providing scaled image information to theuser. This scaled image information can be manually input into the sizeprocessor 114. Alternatively, a micro adjustable reticle can be used formarking and can be coupled with an encoder for providing direct inputsto the processor.

The display 109 can also optionally display the current object distance.As will be described more fully below, displaying the current, in-focus,object distance is useful for making depth and height measurements.

It should be evident from the above description that variouscombinations of the count-to-object distance converter 106, objectdistance-to-magnification converter 107 and size processor 114 arereadily possible. For instance, the formulas or look up tables used toimplement the count-to-object distance converter 106 and the objectdistance-to-magnification converter 107 can be implemented with oneformula or look up table which uses the focus count from the buffer 102as an input and provides the magnification value as an output.Similarly, the object distance-to-magnification converter 107 can becombined with the size processor 114 so that the object distance valuefrom the count-to-object distance converter 106 can be used directly bythe combined block (107/114) in determining the size of the object 110.

Operating the system of the present invention is almost fool-proof.Moreover, the system of the present invention reduces eye fatigue. Asillustrated in the flow diagram of FIG. 13, the user is only required toexecute three simple steps. First, the user must adjust the focus untilthe image on the display screen is focused as shown in step 131. Next,as shown in step 132, the user freezes the focused image. Finally, theuser positions first and second crosshairs on the object being displayedas shown in step 133. Moreover, in the automatically focusing systemdescribed below, the user only has to perform step 133. Accordingly, auser is only required to execute three (or one) simple steps. Thiseliminates many errors that could otherwise be introduced by the user.

The above is an exemplary embodiment of the system of the presentinvention. One skilled in the art can modify the particular componentssuggested without departing from the scope of the invention recited inthe claims. For example, a linear optical encoder for providing theposition of the ocular positioning pin 36 can be used in place of theencoder disk 33 and its optical pickup 30. Such a linear encoder couldbe a plastic or metal strip, having equally spaced slits on it, andbeing mechanically coupled with the ocular positioning pin 36. Anoptical sensor can be used to count light and dark regions as theplastic or metal strip is moved with respect to it. Instead of slots,reflective and non-reflective regions can be used.

Furthermore, an electrical sensor, such as a rheostat, for example, or amagnetic sensor, such as a hall sensor for example, can be used todetermine the position of the ocular lens. However, optical encoders arepreferred because they provide high resolution in a relatively smallpackage.

Similarly, the focusing barrel 39 can be mechanically coupled to theencoder disk hub 34 by gears instead of being directly connected. Also,instead of providing an encoder disk 33 with slits 40, a reflectiveencoder disk with pits can be used. Similarly, instead of incrementingand/or decrementing a count of slits (a relational encoder), the encoderdisk can include coded angular position information (an absoluteencoder).

A fiberscope or videoscope may also be used instead of a rigidborescope. As shown in FIG. 14a, the fiberscope 140 includes a flexibleinsertion tube 141 having a focusing lens system 142 at its distal end.A coherent fiber bundle 143 carries the image to the proximal end of theinsertion tube where a lens system (not shown) provides an image to aviewer. Alternatively, the focusing lens system 142 can be provided atthe proximal end of the coherent fiber bundle 143. The proximal end ofthe fiberscope 140 includes a scope body 144 with a focus controller145. The focus controller 145 can linearly translate a lens of thefocusing lens system 142 by means of a control cable 146 as shown inFIG. 14a or by means of a fine pitch flexible screw 147 and nut 148 asshown in FIG. 14b. In the alternative embodiment having the focusinglens system 142 at the proximal end of the coherent fiber bundle 143,the focus controller 145 can be more directly coupled with the at leastone lens. As shown in FIG. 14c, the encoder 149 may be located at theproximal end of the fiberscope 140 to encode the movement of the focuscontroller 145. However, as shown in FIGS. 14a and 14 b, the encoder ispreferably a linear encoder located at the distal end of the fiberscope140 to encode the linear movement of the lens. Providing the encoder atthe distal end eliminates mechanical position errors due to flexing orstretching in the focusing control cable 146 or backlash in the flexiblescrew 147.

As shown in FIGS. 15a and 15 b, a videoscope 150 has a video camera 151,such as a CCD, adjacent to a focusing lens system 152 at the distal endof a flexible insertion tube 153. The video camera 151 converts theimage to a video signal which is carried to the proximal end of thevideoscope by means of a video signal cable 154. Accordingly, a vieweris not required in a videoscope.

As discussed above with reference to the fiberscope 140, a focuscontroller 155, located at the proximal end of the videoscope 150, canlinearly translate a lens of the focusing lens system 152 by means of afocusing control cable 156 (See FIG. 15a.) or by means of a flexiblescrew 157 and nut 158 (See FIG. 15b.). Also, as discussed above withreference to the fiberscope 140, the encoder 159 may be located at theproximal end of the videoscope to encode the movement of the focuscontroller 155 (See FIG. 15c.) but is preferably a linear encoder 159located at the distal end of the videoscope for encoding the linearmovement of the lens (See FIGS. 15a and 15 b.). In both the videoscopeand fiberscope, if a mechanical means is used to couple the focusingcontrol with the at least one lens, mechanical play in the mechanicalmeans can be compensated for by a processor.

If a fiberscope 140 is used, the system is similar to the system of FIG.10 which uses a borescope as the optical scope 1. However, if avideoscope 150 is used, the viewing means-to-camera adaptor 116, theexternally mounted video camera 103 with CCD 104 are not needed sincethe videoscope 150 includes an internal video camera such as a CCD 151for example.

In alternative embodiments of the scopes and systems of the presentinvention, an automatic focusing device can be used in addition to, orin place of, the focusing knob 38 of the borescope or the focusingcontrols 145 and 155 of the fiberscope 140 and videoscope 150,respectively.

As shown in FIGS. 19a and 19 b, a stepper motor 191 is mechanicallycoupled with at least one lens of a focusing lens system 192, by themeans of a focus control cable or a fine pitch flexible screw forexample. An optional encoder 193 may also be mechanically coupled withthe stepper motor 191 (See FIG. 19a.) or with the at least one lens ofthe focusing lens system 192 (See FIG. 19b.).

The object image is automatically focused with a processor 194 executedprogram. FIG. 18 is a flow chart illustrating the program for automaticfocus. At step 181, the user selects a point of interest via an inputdevice, such as the cursor input control 113 for example. Alternatively,the processor 194 can select a point of interest using a known defectdetection algorithm. Next, a window is defined around the point ofinterest at step 182. The window may be predetermined or may be definedby the user. The window is preferably a rectangle but may be anothergeometric shape. For example, the processor 194 can define a 10 pixel by10 pixel box centered around the point of interest. In a preferredembodiment, the size of the window is limited decrease processing time.

In steps 183 and 184, the stepper motor is placed in an initial positionand the image within the window is sampled. In step 185, the contrast ofthe sampled image is determined in a known way. For example, the averageof the magnitude of the differences in brightness between adjacentpixels can be determined. The higher the average, the higher the imagecontrast. Alternatively, a Fast Fourier Transform (FFT) of the image canbe determined. The higher frequency, the more complex the image and thehigher the contrast.

Step 186 determines whether the contrast determined in Step 185 is amaximum. Since the maximum contrast value corresponds to the “bestfocus,” a focus position value is related to the current stepper motorposition in step 187 when the contrast is a maximum.

Steps 184 through 187 are repeated until a range of stepper motorpositions is complete, as illustrated by steps 188 and 189. If a maximumcontrast is determined at more than one stepper motor position, the bestfocus is selected from either (i) a maximum contrast position nearest tothe focus size, (ii) a maximum contrast position farthest from the focusside, or (iii) an average of the maximum contrast positions. In anembodiment having automatic focusing and a defect detection process, adisplay is not required.

As can be inferred from FIG. 19a, it is possible to eliminate theencoder 193 and base the focus position on the number of steps executedby the stepper motor 191. This would be especially practical if a smallstepper motor could be arranged at the distal end of the insertion tube.If however, the stepper motor 191 is located at the proximal end of thescope as in FIG. 19b, an encoder 193 is preferably included at thedistal end of the insertion tube to eliminate any errors due tomechanical “play” in the mechanical coupling between the stepper motor191 and the at least one lens of the focusing lens system 192.Alternatively, a user may make processor guided manual adjustments.

In a preferred embodiment of the present invention, a focus knob (orfocusing controller) is used with the stepper motor in a “hybrid”focusing operation. A user first uses the focusing knob to coarselyfocus the image. The stepper motor then performs a fine image focusunder control of the processor as described with respect to FIG. 18.Such a “hybrid” operation is advantageous because the range of stepperpositions at which the image is to be sampled and analyzed is decreased.

The system of the present invention can also be used to measure depthand height by determining the difference between the measured objectdistances of two points. For example, the depth of a round-bottomed dingin the surface of a turbine blade can be determined by 1) first focusingthe device of the present invention on the surface of the blade anddetermining the object distance, 2) recording the measured objectdistance to the blade surface, 3) focusing on the bottom of the ding anddetermining the object distance, and 4) subtracting the recorded objectdistance to the blade surface from the object distance to the dingbottom, thereby yielding the depth of the ding.

The above-described procedure can be carried out manually using any ofthe embodiments of the present invention described thus far. Forexample, with the embodiment of FIG. 10, the user can note the objectdistances displayed on the display 109 for the two points of interest(i.e., the blade surface and the ding bottom) and subtract the twoquantities to determine the depth of the ding.

The system of the present invention can provide various features tofacilitate the depth/height measurement procedure. The embodiment ofFIG. 10, for example, can be adapted to provide such features bymodifying the software used to program the system. For example, once theuser causes the system to focus on the first point, e.g., the bladesurface, and has hit image freeze switch 112, the object distance, asdetermined by the count-to-object distance converter 106, is temporarilystored by the software in a register or memory location (not shown).When the user causes the system to focus on the second point, e.g., theding bottom, and hits the image freeze switch 112, the system willsubtract the stored object distance from the current object distance, asgenerated by the converter 106, and display the difference on thedisplay 109.

Optionally, once the first point has been focused and measured, as thesystem is being re-focused and before the freeze switch 112 is pressedagain, the system can display the current object distance, the storedobject distance of the previously frozen image, and/or the differencebetween the two object distances. By updating and displaying, in realtime, the current object distance and/or the difference between thecurrent object distance and the stored object distance, the user cansearch for the lowest point (or highest point) of the feature whosedepth (or height) is to be determined.

The procedure for determining the depth or height of a feature can beautomated even further with the system of the present invention. Forexample, in one embodiment, once the user has aimed the scope at afeature whose depth or height is to be determined, the system can thenautomatically carry out the depth/height measurement procedure. Usingthe automatic focusing procedure described above, the system can focuson and determine the object distance of each of a predefined number ofuniformly distributed points within a window of a predefined sizesurrounding the feature. Once all points within the window have beenmeasured, the system then determines the minimum and maximum values ofthe plurality of object distances measured. The system then determinesthe difference between the minimum and maximum object distance values,which difference represents the depth or height of the feature.

It will be appreciated that when measuring the depth or height of afeature relative to a surrounding flat surface, the scope shouldpreferably be oriented so that its optical axis is perpendicular to thesurface. If this, however, is not the case, at least three points on thesurface must be measured in order to identify the plane of the surface.Once the plane has been identified, the feature's depth or heightrelative to the plane can then be determined. Such a procedure can becarried out, for instance, with the embodiment of FIG. 10 with modifiedsoftware. Using the cursor control, the user can define three points inthe plane surrounding the feature of interest. The system of the presentinvention then automatically focuses on and determines the objectdistance for each of the three points. Using the cursor positioninformation and the object distance calculated for each point, thesystem thereby has three-dimensional coordinates for each point. Thesystem then uses those coordinates to determine the plane of the surfacesurrounding the feature of interest. When the user then positions thecursor on the displayed image of the feature, the system measures theobject distance to the point defined by the cursor and calculates anddisplays the depth or height of that point on the feature relative tothe plane surrounding the feature. The user can then iterativelyposition the cursor on the displayed image of the feature until hedetermines the maximum depth or height, or any intermediate depth orheight of interest. This procedure can also be further automated, asdiscussed above, by programming the system to automatically focus andmeasure multiple points in a window surrounding a feature.

For applications which only call for the measurement of the depth orheight of features, as opposed to their width, a simplification of thesystem of the present invention can be achieved by eliminating thedetermination of the magnification. In this case, only the objectdistance is required.

What is claimed is:
 1. A system for determining a dimension of a feature, the system comprising: a) an optical scope for gathering an image of the feature, the optical scope including i) a focusing device for adjusting a focal position of the image of the feature, ii) a device for detecting a position of the focusing device and for providing a focus position signal based on the position of the focusing device, and iii) an image-to-video converter for producing a video signal of the feature from the image of the feature having its focal position adjusted by the focusing device; b) an image scaling device for providing a scaled image size; and c) a processor, the processor i) converting the focus position signal into at least one of an object distance signal and a magnification signal, and ii) determining the dimension of the feature based on the scaled image size and based on the at least one of the object distance signal and the magnification signal, wherein the dimension of the feature includes at least one of a depth and height of the feature.
 2. The system of claim 1, wherein the processor includes: i) a converter for converting the focus position into an object distance signal; and ii) means for determining the dimension of the feature by determining the difference between a first object distance signal and a second object distance signal.
 3. A system for determining a dimension of a feature, the system comprising: a) an optical scope for producing an image of the feature, the optical scope including i) a focusing device for adjusting a focal position of the image of the feature, ii) a viewer for passing the image of the feature to a plane outside of the optical scope, and iii) a device for detecting a position of the focusing device and for providing a focus position signal based on the position of the focusing device; b) a video camera optically coupled with the viewer of the optical scope and producing a video signal of the feature from the image of the feature; c) an image scaling device for providing a scaled image size; and d) a processor, the processor i) a converting the focus position signal into at least one of an object distance signal and a magnification signal, and ii) determining the dimension of the feature based on the scaled image size and based on the at least one of the object distance signal and the magnification signal, wherein the dimension of the feature includes at least one of a depth and height of the feature.
 4. The system of claim 3, wherein the processor includes: i) a converter for converting the focus position into an object distance signal; and ii) means for determining the dimension of the feature by determining the difference between a first object distance signal and a second object distance signal.
 5. A system for determining a dimension of a feature, the system comprising: a) an optical scope for gathering an image of the feature, the optical scope including i) a focusing device for adjusting a focal position of the image of the feature, and ii) a device for detecting a position of the focusing device and for providing a focus position signal based on the position of the focusing device; b) a processor, the processor i) converting the focus position signal into an object distance signal, and ii) determining a dimension of the feature based on the object distance signal.
 6. The system of claim 5, wherein the dimension of the feature includes at least one of a depth and height of the feature.
 7. The system of claim 6, wherein the processor includes: i) a converter for converting the focus position into an object distance signal; and ii) means for determining the dimension of the feature by determining the difference between a first object distance signal and a second object distance signal. 